1. Field of the Invention
This invention relates to rotary valves for use in conveying systems for powdery and granular materials such as chemical materials, for example, graphite carborundum alundum, etc., and more particularly to a rotary valve of the kind above described having improved pressure shut-off means.
2. Description of the Prior Art
Rotary valves are recently widely used as an essential part of pneumatic conveying systems utilizing air pressure for the conveyance of powdery and granular materials such as chemical materials or as a means for charging or discharging powdery and granular materials such as chemical materials into or from various containers having an internal pressure different from the external pressure.
Prior art rotary valves adapted for applications of the kind above described are generally constructed to shut off gas pressure in the presence of a powdery or granular material being conveyed. Thus, in the prior art rotary valves, the powdery or granular material is inevitably present or trapped in the gap between the valve member and the sealing member which makes a frictional sealing engagement with the valve member. When the powdery or granular materials are hard as graphite, there has therefore been the tendency for the sealing member to be excessively worn resulting in an extreme reduction of the service life of the sealing member. Various means have been proposed hitherto in an effort to minimize this undesirable wear, including slowdown of the speed of swinging movement of the valve member to reduce the rate of wear of the sealing member, and employment of a special steel as the material of the sealing member. However, the former means is defective in that the amount of powdery or granular material being conveyed tends to fluctuate due to the low speed with which the rotary valve is opened and closed. The latter means is also defective in that the proportion of the cost of the rotary valve in the conveying system is increased beyond an economical level.
Another proposal has been made according to which a soft non-metallic material is employed to form the sealing member. However, employment of such a material is also defective in that the sealing member is abraded to an excessive degree due to the wear. The material thus removed is mixed into the powdery or granular material as a foreign matter. pg,3
In order to obviate various defects as pointed out above, it is necessary to resort to a method in which a rotary valve, used for shutting off the flow of powdery or granular material, is disposed separately in the material conveying passage from a gas pressure shut-off valve. According to this method, the flow of powdery or granular material is shut off temporarily by the former valve to establish a material-free atmosphere in the passage, and then the latter valve is actuated to shut off the gas pressure. However, this method is also defective in that equipment cost is extremely high since different kinds of valves must be disposed in series in the material conveying passage.